Evaporative heat exchange and air cooling equipment is well known in the art and includes cooling towers, evaporative condensers, closed-circuit evaporative fluid coolers, direct evaporative coolers, air washers and gas scrubbers, among others. Evaporative heat exchange equipment is typically utilized to provide cooling to a process by rejecting heat from the process or system to the atmosphere. In operation, evaporative heat exchange equipment is utilized to contact heat and water from a process with air. During this contact, heat and mass transfer occur simultaneously, resulting in a portion of the water being evaporated into the air. The energy required to evaporate the water is supplied from the sensible heat of the water, which is not evaporative. Accordingly, the temperature of the non-evaporative water is reduced and cooling has been accomplished. The cooled water is then circulated back to the process wherein it picks up added heat. The heated water is then recirculated back to the evaporative heat exchange equipment for cooling.
The water recirculated through evaporative cooling equipment can become contaminated with impurities during this recirculating operation. These contaminants are introduced to the recirculated water in several ways including for example, air-borne impurities washed out of the air into the recirculated water. These impurities are typically suspended in the recirculated water and can build up over time to possibly clog or block passageways within the fluid system, or further may cause corrosion problems. In addition, dissolved solids in the evaporated water are left behind in the recirculating water during the evaporation process. The situation with regard to the dissolved solids is further exacerbated by the addition of dissolved solids introduced to the system via the make-up water added to replenish evaporated water. As a result of this evaporation and replenishment, the dissolved solids level in the recirculated water can increase to unacceptable levels and cause scaling of heat transfer surfaces and corrosion of system components.
Biological organisms are also being added to the evaporative heat exchange system through the make-up water and from the air passing through the cooling tower. The warm, moist, oxygen rich environment of evaporative cooling equipment represents a planned habitat for biological growth. Microbiological growth typically comprises algaes, slimes, and bacteria. These growths can cause fouling of heat transfer surfaces resulting in reduced operating efficiency, and in severe cases, can completely block passageways within the system.
In order to overcome these entrained contaminants in the recirculated water during operation of evaporative heat exchange equipment, it is typically necessary to treat the recirculated water to maintain its quality within acceptable operating levels. Generally this treatment entails several alternative processes. As an example, the level of dissolved solids present in the recirculated water may be controlled through a process of bleeding off a portion of the water with the highly dissolved solids and thereafter replacing it with fresh make-up water. In those cases where simple bleed and fresh water replenishment is insufficient to control the scaling or corrosive tendency of the recirculating water, specialized scale and corrosive inhibiting water treatment chemicals are utilized. It is generally difficult to effectively and efficiently control microbiological growth within evaporative cooling systems. This difficulty arises in part from the variable conditions present in such systems. This is especially true when evaporative heat exchange equipment is used in comfort cooling, or conditioning systems, which equipment operates a constantly changing condition.
In an air conditioning system for example, the air conditioning is typically required during the warmest period of the day. During this time, the load or the amount of heat to be rejected from the evaporative cooling equipment, is generally at its greatest. As the load increases, the temperature of the recirculated water is also increasing and the volume of air passed through the tower may similarly increase. During such periods, the rate of microbiological growth within the system and the rate of addition of microbiological organisms to the system are at the highest levels. As the outdoor temperatures begin to fall during evening hours and the need for air conditioning decreases, the load on the evaporative cooling equipment and the temperature of the recirculating water also decrease. During such periods, the rate of biological growth and the rate of addition of microbiological organisms are lowered.
In general, biological growth within evaporative heat exchange systems is controlled through the addition of biocide chemicals to the recirculating water. Several different methods typically have been utilized to add biocide to the recirculating water of evaporative heat exchange equipment. The common approach is to pump liquid biocide chemicals from a drum or other container directly into the recirculating water system. This is generally done on a timed basis with a pre-set amount of liquid biocide being added periodically such as on an hourly or daily basis. In some cases, the amount of biocide chemical pumped into the system is done in response to a measured biocide concentration within the recirculating water system. Unfortunately, such automatic biocide feed systems are expensive and require significant human attention to ensure that the proper amount of biocide chemical is supplied to the recirculated water system. In addition, the operator must periodically replace the chemical drum or container when it is empty. Further, in those cases where biocide chemicals are added on a timed basis the biocide chemical is typically overfed during times when the rate of biological growth is slow and underfed when the biological growth rate is high.
An alternative method utilized to introduce biocide chemicals to recirculating water of heat exchange equipment is to pass a side stream flow of recirculating water through a bed of solid biocide chemicals. As the side stream flows through the bed of chemicals, a portion of the biocide chemicals are dissolved into the water stream. However, to prevent the particulate matter contained within the recirculating water from clogging or contaminating the chemical bed, it is typically necessary to filter the side stream water passing through this chemical bed to remove particulate matter prior to its entry into the chemical bed.
A final method utilized to add biocide and other treatment chemicals to recirculating water systems is to manually "hand-dose" or "slug" chemicals to the evaporative heat exchange equipment. Typically, this method is chosen on smaller systems, where the cost of expensive automatic feed systems cannot be justified.
When utilizing this hand-dose method, the operator must manually add large quantities of treatment chemicals to the recirculating water system on a periodic basis, such as once or twice weekly. This method results in erratic corrosion, scaling and microbiological control and can potentially cause serious health hazards when pathogenic organisms are present in the system. As an example, if the time between slug doses of biocide is great enough, and if the pathogen present in the system has a rapid growth rate, the population of such pathogen can increase to potentially dangerous levels between doses of biocide.
Another problem that must be continuously addressed by any microbial control method is the potential of build-up of bio-films and deposits on system components. Biocides are formulated and designed to attack microbiological organisms, which are free in the bulk water. Such biocides are less effective when utilized to control microorganisms contained within bio-films and deposits. This ineffectiveness could be due to the biocides inability to penetrate the deposit or bio-film or due to the fact that the biocides are consumed by reactions in the water phase or at the surface of the film or deposit.
It is important when controlling the above-noted problems in evaporative heat exchange equipment, that treatment chemicals are added in proportion to the need for chemical control within the system and that some level of treatment be maintained within the system at all times. Treatment chemicals should be added in proportion to their need within the system in order that the concentration of treatment chemical within a system be held relatively constant. This would result in the effective control of the microbial populations while utilizing the minimum amount of biocide chemical. In addition, a continual presence of biocide, even at low concentrations, is needed to effectively control microbiological deposits and bio-films.